1. Field of the Invention
This invention generally relates to a roller bearing and particularly to a roller bearing for linear movement of an object. More specifically, the present invention relates to a particular shape of an endless path for rollers of a roller bearing which allows an object to move along a straight line. Still more specifically, the present invention is concerned with an endless path for rollers of a roller bearing for a linear movement of an object, wherein the rollers having the same size in diameter and longitudinal length are arranged in the path with their axes of rotation rotated over 90.degree. alternately with respect to the direction of movement of the rollers or the longitudinal axis of the path.
2. Description of the Prior Art
A prior art roller bearing for linear movement of an object is schematically shown in FIGS. 22 and 23. As shown, a plurality of cylindrical rollers 50 having substantially the same size in diameter and longitudinal length are rotatably retained by a chain-shaped retainer 51 such that the rollers 50 are arranged with their axes of rotation rotated over 90.degree. alternately with respect to the direction of movement of the rollers. An endless path 55 is defined by a straight load path 52, a straight return path 53 in parallel with the load path 52 and a pair of semicircularly curved paths 54, 54 connecting the ends of the load and return paths. It is to be noted that such a roller bearing for linear movement of an object is typically provided in a pair as arranged symmetrically with respect to the center line of linear movement of the object. Thus, as the rollers move along the endless path, the object can move along a predetermined straight path.
However, in the above-described prior art structure, because of the presence of the retainer 51, the number of rollers 50 provided in the endless path 55 is limited. Described more in detail, the fewer the number of rollers 50 in the load path 52, the lower the load bearing capability as a linear motion roller bearing. As a result, the scope of practical applications tends to become limited. On the other hand, if the load bearing capability is increased, the roller bearing tends to become large in size. Moreover, since the endless path 55 of the above-described prior art structure lies on a horizontal plane, there has also been a difficulty in making the roller bearing or its endless path smaller in size.
In order to obviate the above-described defects, there has been proposed a direct contact type linear motion roller bearing having no retainer for a plurality of rollers which circulate along the endless path in direct rolling contact between the adjacent ones with their axes of rotation directed as rotated over 90.degree. alternately with respect to the longitudinal axis of the endless path. In this direct contact type roller bearing, the number of rollers to be provided in the endless path may be increased because of the absence of the roller retainer so that the load bearing capability may be increased correspondingly. Furthermore, in order to make such a linear motion roller bearing smaller in size, in particular narrower in width, it has been proposed to provide the return path obliquely above or below the load path. In this case, there is an increased freedom in designing the connecting curved paths connecting the ends of the load and return paths. For example, the connecting curved paths may be provided to lie in the plane defined by the load and return paths or they may be provided to have a three dimensional shape. In such a case, however, the rollers tend to slip at the joint between the straight, load and return, and curved paths, in particular at the joint between the load path 3 and the connecting curved path 5. With such a slip, which is often called "stick" slip, the resistance of the linear motion roller bearing is higher, so that it is desirable to design the connecting curved paths having a minimum of slip or no slip between the rollers and the endless path.